Was slavery the primary cause of the Civil War? Explain. In any scenario could the Civil War have been avoided?

Slavery per se was not the primary cause for the Civil War; economic reasons were the primary causes. But, slavery certainly played a large part of the Southern economy.

The development of an agrarian economy in the South that was dependent upon slavery and the expansion of an industrial economy in the North that was based on free labor set up the incompatibility of these economies that led to the conflicts which effected the War Between the States. 

Among these conflicts were sectional conflicts over territories in the West which both sections of the country saw as necessary to their continued development: the North needed more areas for the growth of food as its populations grew; the South desired more land for its agrarian economy. There was also the "foot-race between the competing labor systems"; that is, the creation of new slave or free states. This became a contentious issue, for if the states of one labor system (free vs. slave) became larger in number than the other's, the Congress of the United States could pass laws abolishing the system of labor of the smaller block of states.

The North was becoming more and more unwilling to allow the South to expand slavery as there were many who objected to it on moral grounds and others who did not want the West to become slave states for political reasons. Added to this issue, there was the issue of tariffs. Because the North wanted their industries to prosper, it imposed tariffs on products coming into the country. Since the South needed many of the products made by industries of the North or other places, it had to pay tariffs. Therefore, the South tried to stop tariff increases. 

Compromise with the South became less and less likely with the North because of the moral, political and economic issues of slavery and the economic issue of tariffs. In 1854 this Northern opposition led to the formation of the Republican Party, which then supported the interests of the North [Encyclopedia. com]. When Abraham Lincoln was a candidate for president, he supported entirely the economic interests of the North; so, when he became president, the South felt it had no choice but to secede from the Union if it wished to preserve its economic system, a system that included slave labor.

While there are a number of viewpoints on the Civil War, one viewpoint is that this war could have been avoided if Lincoln had allowed to South to secede, an act which was constitutional according to Jeffersonian democracy:

This was a right that continued throughout the initial drafts of the Articles of Confederation, the Constitution and the Declaration of Independence.

But, instead, President Lincoln ordered the military blockade of the ports that belonged to the South, which was actually an act of war.